Today, protection of the environment is of great concern to mankind. Increasing population as well as a general demand for increased quality of life expressed as a healthy and beautiful environment and at the same time a life style based on the use of advanced technology has accentuated the need for water, especially pure water, throughout the world but especially in the industrialized parts of the world.
In highly industrialized countries, especially countries with large urban concentrations, it is necessary to treat the waste water from households and industrial production so as to avoid an unacceptable level of polluted and polluting material in the environment, i.e. in the recipients for the waste water such as lakes, rivers and other waterways, the sea, etc. The polluted and polluting material comprises a variety of substances, for example organic and inorganic substances which may or may not be decomposable in nature. Among the polluting material usually present in waste water effluents, decomposable organic matter and heavy metals are of the greatest concern.
An increasing amount of the waste water which is produced worldwide is now subjected to some kind of treatment, such treatment being of mechanical, chemical or biological nature or any combination thereof. Generally, it is expected that there will be focused even more on waste water treatment in the future as the public awareness of environmental hazards is becoming even stronger than today.
The main purpose of purifying e.g. municipal and industrial waste water is to reduce the content of biodegradable material in the waste water, i.e. to ensure that the treated waste water does not contain such amounts of biodegradable material, i.e. biodegradable organic and/or inorganic matter, that these amounts will lead to an unacceptable low level of oxygen in the recipient due to the amount of oxygen required for aerobic decomposition of degradable (organic) material.
The removal of biodegradable material is often performed by including some sort of biological treatment step in the water purification process. Normally, complex cultures of microorganisms are used to effect the biodegradation (as the microorganisms metabolize the biodegradable material and thereby use it at as source of energy) and the result is a conversion of the biodegradable material into environmentally acceptable compounds such as CO.sub.2 and N.sub.2.
It is especially desired to reduce the amount of organic matter and at the same time to reduce the amount nitrogen-containing components present in the waste water.
Such elimination of nitrogen-containing components from waste water has proved to be difficult and resource consuming. The goal is to convert the nitrogen bound in nitrogen-containing components of waste-water into gaseous (atmospheric) nitrogen, and this is traditionally done by the steps of nitrification (an oxidation step) and denitrification (a reduction step). Prior to these steps, complex nitrogen-containing substances are deaminated by i.a. deaminases produced by the microorganisms (or optionally supplied to the system in question) and the remaining main problem is thus to convert ammonia into gaseous nitrogen.
Numerous attempts of improving the nitrogen elimination in waste water purification has been attempted. The general scheme is the following:
Nitrification: NH.sub.4.sup.+ +O.sub.2 .fwdarw.NO.sub.3.sup.- PA1 Denitrification: NO.sub.3.sup.- .fwdarw.N.sub.2 PA1 or more specifically, the nitrification involves the reaction NH.sub.4.sup.+ +2O.sub.2 .fwdarw.NO.sub.3.sup.- +2H.sup.+ +H.sub.2 O and the denitrification involves the reaction NO.sub.3.sup.- +A.sub.red .fwdarw.1/2N.sub.2 +A.sub.ox, wherein A.sub.red and A.sub.ox are the reduced and oxidized states, respectively, of a compound which is oxidized in parallel to the reduction of NO.sub.3.sup.- to N.sub.2. PA1 A) The biodegradation process is subjected to intermittent aeration, whereby the two processes are substantially non-simultaneous. One example of such processes is described in U.S. Pat. No. 5,304,308. PA1 B) The biodegradation is compartmentalized, in such a way that some compartments have a high oxygen concentration whereas others have a low oxygen concentration. Examples of such processes are disclosed in EP-A-218 289 and in EP-B-233 466. PA1 assessing the value of the at least one metabolic activity parameter (assessed value) in the aqueous medium, PA1 comparing the assessed value with a predetermined range of values or a predetermined single value of at least one metabolic activity parameter which represents metabolic activity of the microorganisms which biodegrade the biodegradable material, the values in the range or the single value being ones which indicate that the microorganisms will perform a simultaneous effective nitrification and denitrification of the biodegradable material contained in the aqueous medium, and thereafter PA1 if the assessed value falls outside the range or is different from the single value, adjusting at least one parameter which has influence on the metabolic activity of the microorganisms in a direction which tends to move subsequent assessed values into the range or towards the single value and ensures that the oxygen concentration in the aqueous medium is kept below 1 mg/l while simultaneous effective nitrification and denitrification take place. PA1 introducing the aqueous medium into a container wherein the biodegradable material contained in the aqueous medium is subjected to biodegradation by microorganisms, and PA1 controlling the biodegradation according to the methods of the invention for the control of biodegradation. PA1 assessing values of the metabolic activity parameter and at the same time assessing efficacy of biodegradation and efficacy of nitrogen removal (as discussed above in relation to the predetermination of values), and PA1 selecting, as the values in the range or as the single value, the values which are associated with simultaneous effective biodegradation and nitrogen removal at oxygen concentrations below 1 mg/l.
Both reactions are facilitated by the microorganisms which are responsible for the biodegradation, but as nitrification is facilitated by high oxygen concentrations and denitrification is facilitated by low oxygen concentrations all methods known to the present inventors rely on one of two principles:
It should be clear that alternative A) is rather time consuming. The prior art processes which use this alternative are furthermore energy consuming, as the supply of oxygen to the system requires much energy for the operation of aeration pumps, means for stirring etc.
Alternative B) overcomes the problem of time-consumption on the expense of space-consumption. The mere fact that there is a spatial distribution of differently aerated zones should make it clear that much space is needed for the process to occur and therefore such processes are mainly used in large-scale water purification. Moreover, the biodegradable material must be transported from one compartment to another in order for the process to be successful, and therefore alternative B) normally requires that means for transportation are present in the system. As for alternative A), the energy requirements are high.
Both of the types of prior art processes suffer further drawbacks:
The mixed cultures of microorganisms which are responsible for the biodegradation are sensitive to changes in their environment. If the oxygen concentration is very low, the composition of the biomass will be adapted in a direction which favours anaerobic processing of biodegradable material, i.e. anaerobic bacteria will be more abundant than in an aerobic environment. The opposite is of course true for situations where the oxygen concentration is very high.
Therefore, in the known processes the composition of the mixed cultures will never or almost never be optimal with respect to neither the process of nitrification nor the process of denitrification, as a certain time is required before the cultures have been adjusted for one of the processes. In other words, large amounts of bacteria do not take part in the process which is currently "desired" at a certain point in time, as they are not capable of performing the process adequately.
Further, when using an operation which requires differences in e.g. aeration (either in time or in space) problems arise with respect to determining for how long the waste water should be processed under each of the two sets of conditions, as the incoming waste water will have to be satisfactorily nitrified in the nitrification phase before the denitrification phase is instituted. In situations were the waste water load is low, no problems will arise, but at maximum loads, the plant has to be dimensioned so that the incoming water can be stored for a sufficiently long period in both steps. In other words, purification plants operated according to the prior art methods have to be scaled for the worst possible situation, i.e. a maximum load, as all material in the polluted water which has been nitrified has to be guided into the denitrification phase.